Methods and apparatuses for printer calibration

ABSTRACT

A test pattern printed by a printer is assessed—without colorimetric equipment—to provide data used in recalibrating the printer. The assessment may be made by an unskilled operator, and can include discerning whether a particular pattern is visible in the printed test pattern, or whether a feature in the test pattern is relatively wider or narrower. From such assessment, needed changes to the printer&#39;s calibration data are inferred and implemented. A variety of other printer calibration techniques are disclosed. The technology is illustrated in the context of dye sublimation printers, and is particularly useful in optimizing printing of digitally-watermarked graphics.

RELATED APPLICATION DATA

This application is a continuation of application Ser. No. 10/954,632,filed Sep. 29, 2004, which claims priority benefit to the following twoprovisional applications: 60/507,568, filed Sep. 30, 2003, and60/514,958, filed Oct. 27, 2003. Each of these patent documents isherein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to printer recalibration, and moreparticularly relates to methods and apparatuses for such recalibrationthat do not rely on calorimetric measuring equipment.

BACKGROUND AND SUMMARY OF THE INVENTION

Printers typically employ stored calibration data that maps input imagesignals into the device's output color space, e.g., to reducenon-linearities in the device's color response. This calibration profileis usually set at the factory, and not thereafter altered.

For many applications, such arrangements are satisfactory. However, inmore demanding print environments—such as high end graphic arts work—aprinter may be recalibrated periodically. Such recalibration can correctfor changes in printer operation due to factors such as changes inambient temperature, non-uniformities of consumables (e.g., inks), anddifferences in printing substrates.

U.S. Pat. No. 6,075,888 describes one technique for recalibrating thestored color profile of a printer. Data corresponding to a series ofinput test colors are provided to the printer, and are mapped to thedevice's output color space using the stored color profile data.Resulting color patches are printed. Colorimetric values of thesepatches are then measured, and the results are compared with the inputtest colors. Differences identified in this comparison are used toadjust the printer's stored color profile, so as to bring thecalorimetric measurements of the printed output and the input testcolors into better agreement.

While such a recalibration procedure may be practical in some settings,it is impractical in others. Among its disadvantages, the foregoingtechnique requires expensive colorimetric measuring equipment, andconsiderable technical expertise. Moreover, it is a prolonged procedure,ill-suited for environments in which regular recalibration may bedesirable.

One setting in which the above-detailed procedure is unsuitable is inconnection with printers used to produce digitally watermarked photo IDcards, such as driver's licenses. For optimal results, it is desirableto recalibrate such printers periodically (e.g., when the printer ribbonis changed), so that the print quality of the resulting ID card isuniformly excellent, and the watermark information is well concealed yetreliably readable. However, the operator of such a photo ID printingsystem is typically a person who is relatively unskilled in printertechnology and colorimetry, and who lacks the time or equipment toengage in a prolonged procedure.

Accordingly, there is a need for a printer re-calibration procedure thatcan be performed quickly, without expensive equipment, and without ahigh level of operator expertise.

In accordance with one embodiment, an operator performs fieldrecalibration of a printer by printing a test graphic on a sample of thetarget substrate, viewing the printed graphic to discern the visibilityof one or more contrasting features, and indicating (e.g., using acomputer user interface) whether or not such features are visible. Basedon the operator's reports of visibility, it can be determined whetherthe correct stored calibration data has been used, and whether it hasbeen tweaked correctly. If not, appropriate adjustments can be made.

In some embodiments, the operator prints and assesses three testgraphics, respectively evidencing the printer's ability to accuratelyreproduce image highlights, mid-tones, and shadows. The operator'sfeedback is used to adjust the stored calibration data so as to betterlinearize the printer's response across the three ranges.

In other embodiments, the observations can be made by a sensor disposedwithin the printer housing.

In accordance with a more general embodiment, a printer is instructed toprint a test graphic comprised of elements that differ slightly in tonevalue. By reference to a difference (or absence thereof) between two ormore of the elements as actually printed, a corresponding change can bemade to the stored printer calibration data.

In one embodiment, the difference is the presence or absence of visiblecontrast between two features in the printed test graphic. In anotherembodiment, there are two visible contrast changes in the test graphic,and the distance therebetween is used in determining a change to thestored printer data.

Another aspect of the invention is a printing system with stored profiledata, and an internal sensor system by which the foregoing difference(s)in actual printed output can be assessed. Still another embodiment is aprinting system that includes a user interface through which operatorassessments of printer performance can be received and used in adjustingthe stored profile data.

One particular embodiment employs a two-step calibration procedure:

(a) normalize the printer and bring it back to a known condition; and

(b) apply dynamic range adjustment, and check if correct table isapplied.

In this embodiment, the software imaging application used with theprinter, or the printer driver, is caused to print a gray balance testtarget that includes several differently-composed (e.g., by different R,G, B values) grey patches. An operator (or a built-in electronic colorsensor) makes a visual comparison to identify the patch that mostclosely matches the color gray on a reference guide shipped with theprinter. The selected patch is identified to a software program. Thisprogram causes the printer to return to a known condition, tweaking thedynamic range adjustment table and thereafter producing correctly colorcalibrated images. To confirm correct calibration, a further testpattern can be printed and again visually inspected.

The foregoing and other features and advantages of the present inventionwill be more readily apparent from the following detailed description,which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of part of a printer, showing the use of alookup table to map input signal values into output signals that drivethe print hardware, to effect desired printer calibration.

FIG. 2 is a plot showing a transfer function of an ideal printer, whereeach increase in input signal value results in a corresponding increasein printed output tone.

FIG. 3 is a plot showing a transfer function of a printer that is out ofcalibration.

FIG. 4 shows a facial portrait produced by a properly calibratedprinter, such as that whose transfer function is depicted by FIG. 2.

FIG. 5 shows a facial portrait produced by a mis-calibrated printer,such as that whose transfer function is depicted by FIG. 3.

FIG. 6 is a plot like FIG. 3, but showing different features.

FIG. 7 is a plot showing a transfer function of an ideal printer thatoutputs uniform print tones over a range of input signal values.

FIG. 8 is a plot like that of FIG. 7, but showing a printer exhibiting anon-linearity like that depicted in FIGS. 3 and 6.

FIG. 9 shows a test card printed on a properly calibrated printer, likethat shown in FIG. 7.

FIG. 10 shows a test card printed on a printer having a transfercharacteristic like that shown in FIG. 8.

FIG. 11 shows a different test card printed on a properly calibratedprinter, like that shown in FIG. 7.

FIG. 12 shows a test card like that of FIG. 11, but printed on a printerhaving a transfer characteristic like that shown in FIG. 8.

FIG. 13 is a block diagram of a printer according to one embodiment ofthe present invention.

FIGS. 14-17 show how different printer transfer response curves can beinferred from different printed test card patterns.

FIG. 18 shows how adjustments in the calibration table vary with inputsignal value.

FIG. 19 shows the transfer function of a different printer that canneither print full black nor full white.

FIG. 20 shows how the transfer function of FIG. 19 may be corrected toyield improved performance.

FIGS. 21 and 22 show still other cards according to aspects of thepresent invention, which can be used for printer calibration.

DETAILED DESCRIPTION

For expository convenience, the following description focuses on anexemplary application of the technology, namely calibrating a dyesublimation printer of the sort commonly used to produce photo IDdocuments, such as driver's licenses. Dye sublimation printers are wellsuited for this application due to the high quality of the printedimages, and the stability of the printed substrates.

Dye sublimation—and its close relative, dye diffusion—are thermalimaging technologies that allow for the production of photographicquality images. Dye sublimation typically employs a set of panels (orribbons) that are coated with a dye (e.g., cyan, magenta, yellow, black,clear-but-UV-responsive, etc.) that can be transferred to a receiversheet or ID document by the application of heat (and sometimes pressure)from a stylus or thermal printhead at discrete points. The dyesublimates and migrates into the document substrate, where it ischemically bound to the substrate or, if provided, to a receptorcoating. Typically, printing with successive color panels across thedocument creates an image in or on the document's surface.

An image can also be imparted via a so-called “mass transfer” (orthermal mass transfer) panel. Standard dye diffusion printers, such asthe model TCP manufactured by Atlantek Inc., and the model Eltron P720manufactured by Zebra Technologies, often incorporate both dye diffusionand mass transfer panels. A mass transfer panel typically includes aresin (e.g., black resin, or a resin having UV properties) that can bethermally transferred to the ID document.

Further details on such printers are provided in U.S. Pat. Nos.5,793,403 and 6,532,032.

To simplify the following discussion, a black and white example isparticularly considered. However, the same principles can likewise beemployed in color embodiments, e.g., by performing such procedures foreach of the component color channels.

Referring to FIG. 1, when image data is provided to a printer 10, theinput pixel values (or other driving signals) are commonly applied to alookup table 12 that maps the input values to output values needed tocause the print hardware 14 to output the desired shade (color). Thus,for example, if a middle gray of value “128” is desired, theidiosyncrasies of a particular printer design may require a drivingsignal of value “126” be applied in order to achieve the desired middlegray output. The lookup table 12 serves to map the input value of 128into an output value of 126.

FIG. 2 shows the result when the lookup table of FIG. 1 is serving itsintended purpose. This chart shows the output printed tone (along thevertical axis) as a function of the input data values applied to theprinter. As indicated by the straight line, the response is linear. Andthe response is such that a driving value of 0 yields black, a drivingvalue of 128 yields a middle gray, and a driving value of 255 yieldswhite.

(The shaded blocks along both axes serve simply to identify gross rangesof the data values, i.e., shadows, mid-tones, and highlights. In actualpractice, the input data values are not grouped into three sets; nor arethe output tones.)

The values in the lookup table 12 are set at the factory and, usually,not thereafter changed. However, factors such as different consumables(inks, print substrates), environmental variables, etc., can cause aprinter's response to deviate from the response shown in FIG. 2.

FIG. 3 shows an illustrative response of a printer that, for somereason, is out of calibration. As can be seen, the response curve is notlinear, but instead is bowed. An input value of 0 still results inprinted black, and an input value of 255 still results in printed white.However, an input value of 128 no longer results in middle gray.Instead, as shown by dotted line “A,” an input value of 128 results in alighter tone (up in the “highlights” range).

The results of this mis-calibration are various. In one respect, it hasthe effect of expanding the shadows in the output image. As shown byrange “B” at the bottom of FIG. 3, the input data values that result inprinted tones spanning the “shadow” range are concentrated down at thelowest values of input data. Slightly higher input data values thatnormally would produce shadow printed tones are now expanded inrendering, producing printed mid-tones.

Likewise, this mis-calibration has the effect of compressing thehighlights in the output image. As shown by range “C” in FIG. 3, some ofthe input data values that normally would produce printed mid-tones arehere rendered as highlights instead. The input data values that normallywould span the full range of highlights now result in output tonescompressed at the top end of the highlight range.

FIGS. 4 and 5 show the result of this mis-calibration. FIG. 4 shows afacial photo as rendered on a properly calibrated printer. FIG. 5 showsthe same photo as rendered on a printer having the response curve shownin FIG. 3.

As can be seen, the mis-calibrated FIG. 5 photo still has full-black andfull-white portions. However, many of the mid-tones are transformed intohighlights; and the intended highlights are largely washed out.

In addition to inferior aesthetics, the mis-calibrated FIG. 5 photo alsosuffers significantly as a carrier of steganographic (e.g., digitalwatermark) information. For best reliability of watermarkcommunications, the tonal variations intended by the input image datashould be faithfully reproduced in the output print.

Referring again to FIG. 3, it can be seen that a wide range of inputsignal values (e.g., 128-255) are rendered in a narrow range of outputprint tones (e.g., mid-highlights and above). This is important inwatermarking because the watermark signal itself is typically of lowamplitude. That is, a watermarked pixel may vary by just a few digitalvalues from the same pixel prior to watermark encoding. If a smallvariation in input signal values is rendered as a further-reducedvariation in output tone values, then the variation may become too smallto detect, and reliability of communication suffers.

Related, in the shadow tones, a small change in input signal value isrendered as an exaggerated change in output tone. This can have theundesirable effect of magnifying the small signal introduced by thewatermarking process, rendering it more visible in the printed output.

Thus, at one end of the visible range, the watermark signal may be tooweak to serve its intended purpose, and at the other end of the visiblerange the watermark signal may be so strong as to become visiblyobjectionable.

The problem is conceptualized in a slightly different way in FIG. 6. Inthis figure, the dashed line indicates the desired, linear, responsebetween input signal values and output print tones. This line has aconstant slope of 45°, indicating that every increment in input signalyields the same increment in output tone darkness. The solid line ofFIG. 6, in contrast, indicates the actual, mis-calibrated printertransfer function.

As can be seen, the slope of the solid line varies over its range. Forinput signal values corresponding to most shadow output tones, the slopetheta_(A) of a line tangent to the transfer function curve is greaterthan 45°. For an input signal value somewhere between light shadows anddarker mid-tones, the slope theta_(B) is 45°. From the middle mid-tonesand above, the slope theta_(C) is less than 45°.

The slope of the curve may be regarded as contrast—the change in printedtone for a given change in input data value. At small input data values(corresponding to intended shadows), the slope is high, and the changein output tone is relatively great for a given change in input data. Atmiddle and larger input data values (corresponding to upper mid-tonesand highlights), the slope is low, and the change in output tone isrelatively small for a given change in input data.

Again, watermark encoding is usually premised on a straight-linecontrast function across all values of input data.

(If the transfer function distortion were known in advance, and known tobe unchanging, the watermark encoding could be tailored to conform tothis distortion, e.g., by reducing watermark signal added to small inputsignal values, and increasing the watermark signal added to large inputsignal values. However, such a known, unchanging transfer function isgenerally not the case.)

FIG. 7 shows another complication. Many printers cannot print 256discrete output tones. Dye sublimation printers of the sort used inprinting drivers licenses, for example, can print only about 32 discretetones. Further gradation is not generally practical due to the physicsof the printing mode. (Inkjets are even worse—typically only being ableto produce 4 discrete output tones, due to limited control of dropletvolume.)

Most inkjet printers, and some other printers, use dithering techniquesto redress this shortcoming, e.g., by alternating between two discreteoutput tones to simulate intermediate tones. Other printers, includingmany dye sublimation printers, do not.

Thus, a dye sublimation printer has a stepped transfer function, asshown in FIG. 7. For input signal values of 0-7, the printer respondswith a full-black output tone. For input signal values of 8-15, theprinter responds with its next-to-black output tone. In like fashion,there are eight different input signal values for each stepped printedoutput tone. The input signal must cross one of these occasionalthreshold values before the output printed tone changes.

Printed output from such a printer thus bands close input signal valuestogether into a uniformly-toned output, as shown by FIG. 9.

FIG. 9 is a test card printed with two stripes. The upper stripe isprinted by uniformly varying the input signal value from 64 to 95.Instead of 32 printed output tones, the stripe consists of just 4 bandedtones: one output for input signal values of 64-71; another for inputsignal values of 72-79; another for 80-87; and the last for 88-95.

The lower strip in FIG. 9 is printed by uniformly varying the inputsignal value from 224 to 255. Again, the same effect is manifested.

(It will be recognized that the tones actually printed in FIG. 9 are notaccurate, but are selected from a limited palette of shading patternsavailable in the drafting tool used. The printed tones for the upperstripe should be much darker (shadows) than the tones for the lowerstripe (highlights). And the printed tone for the range 248-255 iswhite; the dashed border around the area in the lower right of FIG. 9would not actually appear.)

The transfer function of FIG. 7 (and the card of FIG. 9) correspond tothe ideal case, in which the printer's transfer function between inputdriving signal, and output printed tone is linear (albeit stepped). Thetransfer function of FIG. 8 shows the case in which the non-linearityearlier discussed is present.

In FIG. 8, the dashed curve is the transfer function of FIGS. 3 and 6.The solid line is the stepped transfer function of the actual printer.Unlike the ideal case of FIG. 7, the number of input signal values foreach discrete output printed tone is not constant. At low signal values,the number of input signal values per step is small, e.g., 2-4. At largesignal values, the number of input signal values per step is large,e.g., 8-14.

Thus, in the printer whose transfer function is illustrated, inputvalues of 0 and 1 result in an output printed tone of full black (asopposed to input signal values of 0-7 in FIG. 7); input values of 2 and3 result in an output printed tone of next-to-full black (as opposed toinput signal values of 8-15 in FIG. 7). At the other end of thespectrum, input signal values of 242-255 may result in an output printedtone of full white (as opposed to input signal values of 248-255 in FIG.7).

FIG. 10 shows the same test pattern as printed on the card of FIG. 9,but using a printer having the FIG. 8 transfer function. As can be seen,the upper stripe of FIG. 10 here spans five discrete output tones,instead of the four of FIG. 9. That is, instead of eight adjoining inputsignal values producing the same printed output tone, about sixadjoining input signal values produce the same printed output tone. Thewidths of the printed bands are correspondingly reduced.

The lower stripe in FIG. 10 shows the complementary effect for highinput signal values. The lower stripe spans three discrete output tones,instead of the four of FIG. 9. Instead of eight adjoining input signalvalues producing the same printed output tone, about 12-14 adjoininginput signal values produce the same output tone. The widths of theprinted bands are correspondingly increased.

(It will be recognized that the edges of the upper stripe in FIG. 10,and the left edge of the lower stripe, may not be at tonal transitionpoints. That is, the same printed tone may continue for input signalvalues beyond the range printed in these stripes, i.e. “off-the-card.”)

According to one aspect of the invention, information about thedistortion of a printer's transfer function can be inferred by the widthof one or more printed bands on a card like that of FIG. 10.

The width assessment can be absolute or relative. In an absoluteexample, the width of a band in FIG. 10 can be measured (e.g., by aruler). The middle band in the upper stripe, for example, may bemeasured to have a width of about 16 mm. The middle band in the lowerstripe may be measured to have a width of about 24 mm. (In a correctlycalibrated printer, i.e., which produced the card of FIG. 9, all bandsmay have a width of 20 mm.)

In a relative width assessment, a check can be made whether the bands inthe top stripe are wider, or narrower, than the bands in the lowerstripe.

These assessments (which can be made manually by an operator, or by anautomated arrangement), can be used to infer the shape of the transferfunction curve.

One exemplary automated arrangement is shown in FIG. 13, and includes anillumination source and a photosensor to detect changes in contrast inprinted cards (e.g., the contrast change between adjoining bands on thetest cards of FIGS. 9 and 10). A stepper motor moves the card past theprint station in controlled steps. This position data is known to theCPU, which uses the known positions at sensed contrast changes todetermine the width of each printed bands. FIG. 13 also shows a userinterface by which operator assessments of the printed test card(s) canbe entered. (It will be recognized that an arrangement like thatdepicted in FIG. 13 can be included in any prior art printer—equippingit to practice methods according to the present invention.)

Referring to FIG. 14, if the bands in the top stripe are narrower thanexpected, and the bands in the lower stripe are broader than expected,then the transfer function can be inferred to have the general shapeshown on the right side of that figure.

Referring to FIG. 15, if the bands in the both stripes have equalwidths, then the transfer function can be inferred to be linear, asshown on the right side of that figure.

Referring to FIG. 16, if the bands in the top stripe are broader thanthose in the lower strip, then the transfer function can be inferred tohave the general shape shown on the right side of that figure.

Generally speaking, narrow bands indicate too much contrast, i.e., toomuch slope in the corresponding part of the transfer function curve (anda corresponding excess in the angle theta). Conversely, broad bandsindicate too little contrast, i.e., too little slope in thecorresponding part of the transfer function (and a correspondingshortage in the angle theta).

Usually, too-narrow bands at one end of the input signal range areaccompanied by excessively broad bands at the other end of the range.However, this need not always be the case. FIG. 17 shows an example, inwhich the bands are too broad at both ends of the spectrum. In thiscase, the transfer function in the middle range of input signal valuesis steep, with a large angle θ.

It will be recognized that the test cards shown in FIGS. 14-17 aresomewhat incomplete, in that they don't provide printed output except inthe range of input signal values 64-94 and 224-255. However, in otherembodiments, broader ranges of input signal values can be tested, e.g.,by using more printed test stripes. Or one or more stripes can beprinted spanning a larger range of input signal values (e.g., in thelimiting case, spanning 0-255). Multiple cards may be used, or all thestripes can be printed on a single card.

One alternative test card includes three stripes—one centered around themid-point of the shadow range (e.g., centered around an input signalvalue of about 42), one centered around the mid-point of the mid-tonerange (e.g., centered at about 128), and one centered around themid-point of the highlights range (e.g., centered at about 214). Eachrange could extend to meet the adjoining range (e.g., each spanningabout 85 digital numbers). Or the range could be shorter or longer.

Desirably, the range is not so short that an edge of a band of interest(i.e., the border at which the contrast changes with steps in the outputtone) is “off-the-card.” For example, if broad bands—each correspondingto up to 14 different input signal values—are anticipated, then a rangeof 25 input signal values may be too small. One band may be partiallyoff the left edge of the card, and have 12 values “on the card.” Theadjoining band may start with 13 values “on the card” and terminate withanother value off the right edge of the card. Neither band ismeasurable, since each ends off the card.

In preferred embodiments, each stripe spans a number of input signalvalues at least equal to 2N+1, where N is the broadest band anticipated(measured in range of corresponding input signal values).

Once the shape of the transfer function has been inferred (which may beby the foregoing procedure, or otherwise), then the calibration datastored in the printer can be changed accordingly.

In one embodiment, the printer has an interface through which datacharacterizing the shape of the transfer function is received. If theassessments are made by a human operator, this interface can be agraphical user interface, such as a display on a screen with which theuser interacts by means such as typing, touching, or mouse clicking,etc. If the assessments are made by one or more sensors within theprinter, the interface can be an electrical or data interface.

The adjustments to the stored calibration data can be effected invarious ways. One is by adding (or subtracting) small values (e.g., 1 to10) to the calibration data already stored (e.g., in lookup table 12 ofFIG. 1). Another is by multiplying the existing calibration data valuesby corresponding scale values (i.e., values close to 1, such as 0.85 to1.15). Still another is by maintaining the original calibration data,and effecting the changes in another lookup table that is seriallyinterposed before or after the table in which the original calibrationdata is stored. The input data can thus be pre-compensated beforeapplication to the table 12 of FIG. 1, or the output data from table 12can be post-compensated through such a table. (In all these cases,iterative adjustment and re-testing can be employed.)

The adjustments can be tailored to precisely correspond to the assessedprint characteristics, or rote adjustments can be employed.

In the latter case, the shape of the actual transfer function may befirst identified as either of the form shown in FIG. 14 (i.e., the bandsin the upper stripe are narrower than those in the lower stripe), or ofthe form shown in FIG. 16 (i.e., the bands in the upper stripe arebroader than those in the lower strip). If the test card indicates asituation like that shown in FIG. 14, the lookup table values shouldgenerally be decreased—with the largest decrease at the middle of therange, tapering to nil change at either end. This is shown in thefollowing tables, with the original values in parentheses and replacedwith adjusted values:

TABLE I Input Value Output Value 32 (32) 30 64 (64) 61 96 (96) 92 128(128) 123 160 (160) 156 192 (192) 189 224 (224) 222Conversely, if the actual curve is found to be of the general shapeshown in FIG. 16 (i.e., the bands in the upper stripe are broader thanthe bands in the lower stripe), the lookup table values should generallybe increased—with the largest increase at the middle of the range,tapering to nil change at either end. This is shown in the followingpair of lookup tables, with Table III being the lookup table prior toadjustment, and Table IV being the table after adjustment.

TABLE II Input Value Output Value 32 (32) 34 64 (64) 67 96  (96) 100 128(128) 133 160 (160) 164 192 (192) 195 224 (224) 226

These changes may more readily be understood by reference to FIG. 18(which corresponds to the FIG. 14 form of distortion). It will be notedthat the largest discrepancy between the idealized straight-linetransfer function and the actual transfer function is in the range ofmiddle input signal values, with the difference tapering to zero at theends. If the desired output tone is indicated by the arrow A, thisoutput would ideally be produced by an input signal value B. However,due to the non-linearity of the printer, it is actually an input signalvalue C that produces this desired output tone A. To linearize thecurve, the lookup table output value formerly found for input value Cshould be substituted at input value B, i.e., a reduction in the amountshown by the arrow D.

The rote adjustment noted above, based on inferred transfer functioncurve shape, can be applied and another test card then run using theadjusted printer. If the second test card still has the appearance ofFIG. 14 (i.e., with the upper bands being narrower than the lower bands)then the same adjustment can be applied again. This procedure can berepeated until the band widths in the upper and lower test stripes areapproximately equal.

In another arrangement, a compensation particularly tailored to theobserved test results can be applied. For example, referring to FIG. 14,it will be recognized that the absolute width of the band for an inputsignal value of 72 is related to the slope of the printer's transferfunction at this value. By measuring the widths of the bands arounddifferent input signal values, a relatively precise inference of therequired adjustments to the lookup table data can be made. (Toaccurately adjust the lookup table, it is best to print test stripsencompassing the full range of input signal values so that the printeroperation is characterized over its full range of operation.)

If the printer interface is provided with data indicating the measuredwidth of each of the bands for input signal values ranging from 0 to255, the lookup table data can be precisely corrected. Thus, if thefirst band (i.e., input signal values of 0-7) has a width of 10 mm., andis expected to ideally have a width of 20 mm., then it can be seen thatthe slope of the transfer function is twice the correct value (i.e., thewidth of the band is directly related to slope of the transferfunction). To compensate for this, adjustments per the following tablemay be applied (again, these adjustments can be made by subtracting fromthe values earlier in the table, or by multiplying the original tablevalues by scaling factors, or by pre- or post-compensation tables,etc.):

TABLE III Input Value Output Value 0 0 1 (1) 0 2 (2) 1 3 (3) 2 4 (4) 2 5(5) 3 6 (6) 3 7 (7) 4 . . . . . .

The same assessment, and correction, can be performed for eachsuccessive band of input signal values.

Another type of adjustment is somewhat more than rote, but somewhat lessthan fully precise. This type of adjustment is based on the number ofbands appearing in the top and bottom test stripes. In FIG. 14, thenumbers are 5/3. In FIG. 15 it is 4/4. In FIG. 16 it is 3/5. In FIG. 17it is 3/3. Each of these pairs of numbers can invoke an associatedadjustment in the compensation table. The more bands in each stripe, thesteeper the actual transfer function curve in the corresponding portionof the input signal range. Conversely, the fewer bands in each stripe,the more gradual the actual transfer function curve. Knowing therelative steepness of the curve in different regions, correspondinglydifferent adjustments can be applied to the printer calibration data.

A different test card is shown in FIGS. 11 and 12. The FIG. 11 card wasprinted on a correctly calibrated printer; the FIG. 12 card was printedon a printer whose transfer function is like that shown in FIG. 8.

In both cards, the background appearing in the top three tiles isprinted using an input signal value of 82. The letters A, B and C areprinted using input signal values of 80, 78 and 70, respectively. (Theseparticular numbers are somewhat arbitrary, but serve to illustrate someof the operative principles.)

Consider first the tile in the upper left of FIG. 11. Recall that in acorrectly calibrated printer, input values of 80-87 all produce the sameprinted output tone. (Likewise, input values of 72-79 all produce thesame, slightly darker, tone, and input values 64-71 all produce thesame, still darker, tone.) Thus, the letter A (printed with input value80) should not visibly contrast with the surrounding background area(input value 82), since both should fall within the same output toneband. But letter B (printed with input value 78) should be visible, asit should fall in a different output tone band than the background (82).Even more apparent should be letter C, since it is printed with value 70that is in a different, and not even adjoining, output tone band

Likewise in the lower tiles of FIG. 11. The background is printed usingan input signal value of 242. The letters D, E and F are printed withinput signal values of 240, 238 and 230, respectively.

Again, in a properly calibrated printer, the letter D should not bevisible. That is, the input signals that form the letter D (value 240)should produce the same output tone as that produced by the inputsignals corresponding to the surrounding background (value 242).However, the letter E should be visible, as the input signal thatproduced it (238) should produce a different output tone than thebackground signal (242). And letter F should be even more visible, sinceit is printed with a value (230) that yields a tone falling in adifferent, and not even adjoining, band than the background (242).

Thus, if a test card of the type just detailed is printed on an ideallycalibrated printer, then it should have the appearance of FIG. 11, withthe letters A and D not visible, and the other letters in each rowhaving progressively more visibility.

FIG. 12 shows the same card printed on a printer having the transferfunction detailed in FIG. 8. Recall that in this mis-calibrated printer,input signal values corresponding to shadow tones are printed with toomuch contrast. That is, the output shadow tone bands do not correspondto 8 input signal values each (as in the ideal case), but insteadcorrespond to only 2 to 4 input signal values. So in this case, insteadof the letter A input signal (value 80) falling within the same outputtone range as the surrounding background (input signal value 82), itfalls within a different output tone range. Accordingly, the A contrastswith the background and becomes visible. As before, letters B and C arevisible. (In this case, letters B and C are even more apparent thanbefore, since they fall in tone bands further spaced from the backgroundtone, due to the high contrast of the mis-calibrated printer in theseshadow tones.)

The mis-calibration yields a different result in the lower 3 tiles ofthe FIG. 12 test card. Here, as before, the letter D (input signal value240) produces an output tone identical to the surrounding background(input signal value 242), and is thus not apparent. However, in thiscase the letter E is also not apparent. This is because the printer'smis-calibration causes its input signal value (238) to be rendered asthe same output tone as the surrounding background area (242)—contraryto the printer's ideal operation. Letter F is visible in FIG. 12, butonly barely. While the printer has rendered its input signal value (230)in a different output tone than the surrounding background (242), it isnot as visibly distinct as in the FIG. 11 test card, because the letterand the background are rendered in adjoining output tones.

Thus, by examining a test card like that detailed above, certainmis-calibrations of the printer can be inferred. If the letter A isvisible, then this generally means that the printer provides too muchcontrast in the shadows (i.e., the transfer function curve is too steepin this area). Likewise, if the letter E is not visible, then thisgenerally means that the printer provides too little contrast in thehighlights (i.e., the transfer function curve is too gradual in thisarea).

Moreover, although not particularly illustrated, it will be recognizedthat if the letter B is not visible, then this generally means that theslope of the transfer function is too gradual for input signal valuesthat are intended to produce shadows (i.e., too little contrast in thelower values of input signals). Conversely, if the letter D is visible,then this generally means that the slope of the transfer function is toosteep for input signal values that are intended to produce highlights(i.e., too much contrast in the higher values of input signals).

(The reference to “generally” in the foregoing paragraphs is anacknowledgement that the visibility of a letter on the FIG. 12 testcard—or lack thereof—may not be due to too much or too little localizedslope in the transfer function. Instead, this effect could be present ina printer with a generally linear response characteristic, e.g., if thetransition between pure black and next-to-black occurs between inputsignal values of 6 and 7, instead of between 7 and 8. A single suchmisplaced transition boundary could cause some apparently atypicalbehavior through parts of the input signal range that are highly linear.This is another example of uncertainty that can be addressed—ifdesired—by a more thorough evaluation of the printer performance, e.g.,by using test cards that involve many more input signal values than thefew shown in FIG. 12.)

Again, it will be recognized that test cards of the sort just-discussedcan be assessed by a human operator, and the observed results enteredinto a user interface (e.g., “Type in the letters that are visible onthe card”). The printer CPU can respond to these assessments to adjustthe lookup table data as earlier described. Alternatively, the sensingof the characters can be done with one or more electronic sensors (asindicated in FIG. 13), and this data again used to drive correspondingchanges in the lookup table values.

While the foregoing examples depict a printer whose distorted transferfunction encompasses the full range from black to white, the sameprinciples can be applied to any printer. Consider a printer whoseactual transfer function is shown by FIG. 19. This printer cannot outputa true black (perhaps due to the substrate's resistance to acceptingdye) and cannot output a true white (perhaps due to a tinting of thesubstrate color). Nonetheless, its printing can be improved—especiallyfor watermarking purposes—by linearizing the range of responses that theenvironmental factors (e.g., substrate characteristics) permit. Thus,the transfer function of FIG. 19 might be corrected to that shown inFIG. 20. Likewise, it is not a requirement that the printer have arelatively simple transfer function of the sort depicted in FIGS. 13, 15and 16. These were chosen to facilitate explanation. The same principlesare applicable to printers having any form of transfer function curve.

FIGS. 21 and 22 show yet other forms of printer calibration cards. Thesecards can be used by printer operators to re-calibrate printed hues,e.g., whenever printer consumables (e.g., inks, transfer ribbons, orsubstrates) are changed. (Changing any of these consumables may triggerthe printer to present a prompt to the user requesting or requiringrecalibration.)

The card 90 of FIG. 21 is comprised of plural rectangular patches 92.Each is printed with a color tone defined by different R-G-B values. Thetriplet of numbers (e.g., 42/56/50 in the upper left patch) representsthe percentage of red, green, and blue. In the printer earlier detailed(with each color ranging in value from 0 to 255), the 42% corresponds toa green value of 0.42*255, or 107; 56% corresponds to a red value of143, and 50% corresponds to a blue value of 128.

In actual practice, the triplet of values is not printed on the card;they are shown in FIG. 21 for illustrative purposes only. Instead, eachpatch 92 is of a uniform color (optionally, with an identificationindicia printed therein).

The card of FIG. 21 thus presents a spectrum of 60 colors. The patchnear the center, with the bold outline, is intended to be printed inmid-gray, i.e., 50% values of red, green and blue. Those to the left ofthis patch have decreasing amounts of red (2% less in each successivecolumn, in the illustrated card). Those to the right of this patch havesimilarly increasing amounts of red. Those below this patch havedecreasing amounts of green (3% less in each successive row, in theillustrated card). Those above this patch have similarly increasingamounts of green.

It will be recognized that the color blue is said to be constant at 50%in the patches of the FIG. 21 card. Desirably, however, since the valuesof colors red and green change, the proportion of blue to these colorsshould also change. That is, in a more preferred arrangement, blue isrelatively more dominant in patches to the upper left, and is relativelyless dominant in patches to the lower right.

FIG. 22 shows a companion card 94 comprising a reference patch of truegray 96 (i.e., 50/50/50). Typically, this card is provided by theprinter vendor and is not printed by the operator at the time ofrecalibration. Formed through card 94 is a hole 98.

In use, companion card 94 is slid by the operator over test card 90until the color of the patch 92 showing through the hole 98 most closelymatches the reference gray color 96 on the companion card. The referenceindicia of the patch 92 containing the best-match color is then typed(or otherwise) entered into a user interface.

Imagine that the operator types “10” into the user interface, signalingto the printer that the patch in the upper left of the card has a colormost closely matching the reference gray of the companion card 94. Theprinter CPU responds by making appropriate changes to lookup table datain the printer (e.g., changes of the sort discussed earlier).

In this case, the R-G-B values associated with patch 92 are 42/56/50. Asto the 42% red, the printer actually rendered this patch with about 50%red (since patch 92 is closest to true mid-gray). Accordingly, theprinter as originally calibrated was printing red too strongly (i.e.,the printer's red transfer function may appear like the curve to theright of FIG. 14). To redress this, a lookup table adjustment is made toreduce the amount of red printed at the middle of the transfer functioncurve. (As before, the maximum adjustment may be made towards the middleof the curve, with tapering amounts of adjustment made towards eitherend.)

The green value of patch 92 is 56%. The printer as originally calibratedprinted this patch with a green component more like 50% (since thispatch was found to be closest to true gray). Accordingly, it appears theprinter did not apply as much green to the printed output as the drivingsignals intended (i.e., the green transfer curve may have a shape likethat shown at the right side of FIG. 16). Again, a corresponding changecan be made to appropriate lookup table values, e.g., to increase theamount of green applied at middle values, with tapering changes appliedat greater and lesser green values.

As noted, all patches in the FIG. 21 card are desirably arranged to haveapproximately the same luminance, so when red and green are bothincreased, blue is decreased to keep the luminance constant. Similarlywhen red and green are both decreased, blue is increased to keep theluminance constant.

More than one card may need to be printed with different levels ofluminance, if the change in consumables causes a luminance shift. If ascanner is being used, several different luminance levels (say 4) couldbe printed on the same card and the luminance and color shift determinedtogether.

The foregoing procedure improves accuracy of printer response in themiddle of the color transfer functions. If desired, similar test cardscan be made and used to calibrate color contributions in the shadows(e.g., patches centered about 20%, 20%, 20%, compared against a truedark gray patch with these values) and in the highlights (e.g., patchescentered about 80%, 80%, 80%, compared against a true light gray patchwith these values).

Recall that the printer earlier detailed does not render 256 tones ofany color. Instead, only 32 gradations of each color are possible. Inprinting the test patches on card 21, dithering may be used to obtainintermediate color values. Although the printer may not nominallyprovide dithering capability, dithering techniques can be employed inestablishing the pixel values that make up each of the patches 92.

In an illustrative embodiment, the known Floyd-Steinberg successiveerror-dispersion dithering technique is used. Thus, if a red value of242 is desired, and the nearest values the printer can render are 240and 248, then patch 92 can comprise an array of pixels in which 75% havea red value of 240, and 25% have a red value of 248—interspersed inregular fashion. Thus, the print file provided to the printer comprisesjust these 240 and 248 values.

The foregoing examples are naturally subject to myriad variations. Inthe test card of FIG. 21, for example, it will be recognized that theincrements by which the colors change in successive rows/columns can betailored to particular applications; the 2% and 3% numbers are notcritical. Nor is it essential that all patches in a row or column shareone or more common value.

Although calibration was described with reference to tests based on red,green and blue, tests using other color spaces can of course be used(CMYK being among the alternatives).

The techniques just described can be used in combination, or in hybridform, with those earlier described—providing increased calibrationaccuracy.

It should be recognized that the principles discussed above areapplicable in contexts other than those particularly detailed. Thus, itis not essential that the invention be employed with dye sublimationprinters; any printer can employ these principles. For example,excellent quality photo ID cards can also be produced by using ink jetprinting to print on a substrate sheet, e.g., a Teslin® sheet. The inkjet printed substrate is then preferably over laminated with, e.g.,polyester laminates and then cut into a typical ID card size (e.g.,conforming to an ISO standard). Other printing technologies, includingcolor xerography, offset press, laser engraving, etc., can likewisebenefit by application of the present technology.

Moreover, the example of an input signal that ranges in value from 0 to255 is exemplary only, not limiting. Similarly, there is nothingessential about the print mode being able to produce only 32 distinctoutput tones. Although the illustrative printer does not employdithering or other known image enhancement techniques, embodiments ofthe invention can use such techniques if desired. (In some suchembodiments it may be desirable to temporarily disable dithering whenprinting certain test patterns, so the transfer response transitions canbe more readily discerned.)

Of course, it will be recognized that the techniques detailed hereinfind application in myriad printing applications, not just in theproduction of digitally watermarked photo ID cards.

Not much has yet been said about the details of digital watermarking.Such technology is well known in the printing art. The assignee's U.S.Pat. No. 6,614,914 is exemplary. Some of my earlier work in this fieldis detailed in U.S. Pat. No. 6,700,995 (“Applying Digital WatermarksUsing Dot Gain Correction”) and published application US 2002-0164052(“Enhancing Embedding of Out-Of-Phase Signals”). Other applications thatparticularly discuss watermark technology as it relates toidentification cards (e.g., drivers' licenses) include 60/495,373, filedAug. 14, 2003 (“Identification Document and Related Methods,” whichserved as a priority application for published document US 2004-0181671)and US 2003-0183695 (“Multiple Image Security Features forIdentification Documents and Methods of Making Same”). The teachings ofeach of these documents can be employed in embodiments according to thepresent invention. These patent documents, as well as those earliercited, are incorporated herein by reference.

Having described and illustrated the principles of my inventive workwith reference to illustrative embodiments thereof, it will berecognized that these embodiments are illustrative only and should notbe taken as limiting the scope of the invention. Rather, I claim as myinvention all such modifications as may fall within the scope and spiritof the following claims, and equivalents thereto.

1. A method of calibrating the method comprising: instructing a printerto print a test graphic comprised of features, wherein each feature hasa different tone value; receiving data based on an assessment of theappearance of the test graphic, as actually printed; and changing atonal compensation parameter of the printer based on the received data;wherein the received data relates to a width dimension of one or more ofthe features.
 2. The method of claim 1, wherein the received dataindicates a number of bands printed in a region of the test graphic. 3.The method of claim 1, wherein the received data indicates whether afirst of the features is wider or narrower than a reference width. 4.The method of claim 1, wherein receiving data based on an assessment ofthe appearance of the test graphic comprises receiving data from asensor of the printer.
 5. The method of claim 1, further comprisingprinting a digitally watermarked document using the printer afterchanging of the tonal compensation parameter.
 6. The method of claim 1,wherein the received data relates to a width dimension between a firstchange in contrast and a second change in contrast in the test graphicas actually printed.
 7. A method of calibrating, the method comprising:printing, with a printer, a test pattern, the test pattern including afirst visible feature that spans a plurality of pixels in both width andheight; assessing a width of the first visible feature; and modifying atonal compensation parameter of the printer based on the assessment. 8.The method of claim 7, wherein the printer comprises at least one of anink-jet, dye sublimation, xerography, offset press, mass transfer, andlaser engraving printing apparatus.
 9. The method of claim 7, whereinassessing the width of the first visible feature comprises receivingdata from a sensor of the printer.
 10. The method of claim 7, furthercomprising printing a digitally watermarked document using the printerafter modifying the tonal compensation parameter.
 11. The method ofclaim 7, wherein assessing the width of the first visible featurecomprises determining if the width of the first visible feature isgreater than a reference width.
 12. The method of claim 11, whereinassessing the width of the first visible feature comprises determiningif the width of the first visible feature is greater than a width of asecond visible feature.
 13. A method of calibrating, the methodcomprising: printing, with a printer, a test pattern; receiving dataindicating one of two possible and opposite assessments concerning theprinted test pattern; making a first rote change to a tonal compensationparameter of the printer if the received data indicates a first of theassessments; and making a second, different, rote change to the tonalcompensation parameter if the received data indicates a second of theassessments.
 14. The method of claim 13, wherein the printer comprisesat least one of an ink-jet, dye sublimation, xerography, offset press,mass transfer, and laser engraving printing apparatus.
 15. The method ofclaim 13, further comprising receiving the data from a human operatorviewing the printed test pattern.
 16. The method of claim 13, furthercomprising printing a digitally watermarked document using the printerafter changing the tonal compensation parameter.
 17. The method of claim13, wherein the data comprises information provided by a sensor of theprinter.
 18. The method of claim 13, wherein the two possibleassessments comprise an assessment that a feature printed in the testpattern can, or cannot, be discerned.
 19. The method of claim 13,wherein the two possible assessments comprise an assessment that a firstfeature printed in the test pattern is relatively wider, or relativelynarrower, than a reference width.
 20. The method of claim 19, whereinthe reference width comprises a width of a second feature printed in thetest pattern.
 21. A method comprising: printing, with a printer, a testpattern comprising a plurality of patches, each patch being defined by aunique combination of colorant data signals; receiving an indication ofwhich patch appears to most closely match a reference color; andmodifying a tonal compensation parameter based on the indication;wherein certain ones of the plurality of patches are defined by uniquecombinations of three different colorant signals.
 22. The method ofclaim 21, further comprising placing a medium over the printed testpattern, wherein: the medium has the reference color printed up to anedge of the medium; the edge of the medium is configured to define aboundary between the reference color and one or more patches of theprinted test pattern; and the medium is configured to aid in identifyinga patch that appears to most closely match the reference color.
 23. Amethod comprising: instructing a printer to print a test graphiccomprised of elements that should differ slightly in tone value;receiving, from a sensor of the printer, data based on a differencebetween the visual appearance of a first and second of the elements, asactually printed; and changing a tonal compensation parameter of theprinter based on the received data.
 24. The method of claim 23, furthercomprising printing a digitally watermarked document using the printerafter changing the tonal compensation parameter.
 25. A methodcomprising: printing, with a printing apparatus, a test patterncomprising a plurality of patches, each patch being defined by a uniquecombination of colorant data signals; receiving an indication of whichpatch appears to most closely match a reference color; and modifying atonal compensation parameter based on the indication; wherein theplurality of patches comprises three patches having different values ofa first colorant data signal, and three patches having different valuesof a second colorant data signal.
 26. An apparatus for calibration, theapparatus comprising: a processor configured to: instruct a printer toprint a test graphic comprised of features, wherein each feature has adifferent tone value; receive data based on an assessment of theappearance of the test graphic, as actually printed; and change a tonalcompensation parameter of the printer based on the received data;wherein the received data relates to a width dimension of one or more ofthe features.
 27. A non-transitory computer-readable storage mediumhaving instructions stored thereon that, if executed by a computingdevice, cause the computing device to perform operations comprising:instructing a printer to print a test graphic comprised of features,wherein each feature has a different tone value; receiving data based onan assessment of the appearance of the test graphic, as actuallyprinted; and changing a tonal compensation parameter of the printerbased on the received data; wherein the received data relates to a widthdimension of one or more of the features.